GY 111 Lab Notes D. Haywick (2006-07) 1 GY 111 Lab Lecture Note Series Lab 1: Introduction to Mineral Identification Lecture Goals: A) Definitions B) How to identify minerals C) Physical Properties Reference: Haywick (2001). GY 111; Earth Materials Lab Manual; Chapter 1. A) Definitions This will be a long lecture. It’s purpose is to introduce the basic tests and mineral properties that you will have to understand in order to identify minerals and later, rocks. Next week you will be able to “play” with the specimens. For today, just sit back and enjoy the ride. You need to know two definitions at this time: Mineral: a naturally occurring, inorganic, solid substance with a well-defined crystal structure and a unique chemical composition Rock: a naturally occurring substance containing 2 or more minerals. In this class, you will eventually learn how to identify both rocks and minerals. We usually start off with minerals because they have more ordered properties (i.e., they are easier to identify using a flow- chart methodology). B) How to identify minerals The first thing you have to do is distinguish between a rock and a mineral. If the substance looks relatively pure (e.g., it contains a single material or looks homogeneous), it may be a mineral. If it looks like an agglomerate of stuff (multi-coloured or different types of reflectance), it is likely a rock. Beware! Some minerals display multiple colors or are slightly stained by weathering rinds. Do not get confused by color masking. So what are some examples of minerals? The most common around here is quartz (image to right from www.csm.jmu.edu). This substance is solid, naturally occurring, inorganic, has a unique chemical composition (SiO2) and has a well defined crystal structure (hexagonal shaped crystal -bi-pyramidal). Here are some other substances worth discussing. Fluorite (CaF2) Gold (Au) Gypsum (CaSO4.2H2O) Shells (CaCO3) Chert (SiO2) Opal (SiO2.H2O) High School Ring Ruby (Al2O3) GY 111 Lab Notes D. Haywick (2006-07) 2 The first 4 are all minerals. The last 3 are not. Many rubys are today synthetic, so they do not meet the naturally occurring requirements. Chert and opal are naturally occurring, but they do not have a crystalline structure. They are like glass and as such, do not meet the definition of a mineral. We refer to these substances as mineraloids. Other mineraloids include limonite (FeOOH) and bauxite (Al2O3). Were you surprised by shells being classified as minerals? Although organically produced, the carbonate minerals that form shells followed inorganic pathways. Some people (e.g., your humble instructor) like to refer to these substances as biogenic minerals. The most important ones are aragonite and calcite (both CaCO3) and silica. Identification of minerals is relatively easy. All that you have to do is recognize the properties that characterize them. You distinguish people based upon their appearance, and minerals are identified in the same way. With people, you use height, hair color, weight, sex, age etc. to distinguish your friends. With minerals, its color, luster, specific gravity, cleavage, crystal habit etc. Incidentally, some of these properties are more readily recognized and remembered than the properties you use to identify people (e.g., how old is your father?). Now for the scary stuff. At last count, there were over 4000 separate minerals. Luckily, there are really only 30 or 40 that are important rock-formers and of these, only 20 or 25 are really common. So you really do not have to learn too many. Expect 40-45 minerals in your sample tray. Now for the demonstration: Property 1: Colour (sulfur, malachite, fluorite, quartz varieties) Mineral 1: sulfur (S): Sulfur is easy to remember because of its color. Color is an important property, but it is not always reliable. For sulfur, it is a diagnostic property. The color in sulfur is caused by the atomic structure and its chemical composition. It is always the same. When colour is a diagnostic property, it is said to be idiochromic Mineral 2: quartz (SiO2), fluorite (CaF2). Quartz comes in many different shades. Each of the different colours of quartz is given a specific sub-name (variety). They include rose, citrine, smoky, milky, amethyst, crystal. Despite the color variation, all of the varieties are the same mineral. The different colours are induced by minute quantities of foreign substances like dust, water or trace elements. Even radiation causes color changes. For quartz and a large number of other minerals (including calcite and fluorite; image to right from www.icminerals.com), color is non-diagnostic and the color is said to be allochromic. Minerals are largely separated on the basis of overall colour (i.e., dark vs. light), but sometime they "fool" you (e.g., dark quartz, light quartz). A better property to describe mineral color is streak. Property 2: Streak (quartz, sphalerite, hematite, pyroxene) Streak is the colour of a mineral when powdered. Most light-coloured minerals produce a white or colourless streak. Some however give diagnostic streaks: Sphalerite (ZnS): pale yellow Hematite (Fe3O4): red-brown GY 111 Lab Notes D. Haywick (2006-07) 3 Try to streak all mineral specimens, but be careful to choose clean surfaces. Contamination can miscolor your streak. Property 3: Specific Gravity (quartz, galena, celestite) The apparent mass (or weight) of a specimen is also a useful property in the identification of minerals. The only problem is that weight is determined by both the density of a mineral and the size of the specimen (don't believe what you hear - size does matter). Minerals that are more dense will weigh more than minerals of similar size that are less dense. Density is largely controlled by the mass of the elements that comprise the mineral. Quartz is composed of one part Si to 2 parts oxygen (both are relatively light elements according to the periodic table). In contrast, galena which is composed of 1 part lead to 1 part sulfur is a heavy mineral (locate the position of lead on the periodic table of the elements). Density is expressed in units of g/cm3 as it is a ratio of mineral's weight to its volume (this is how you get around the different sizes of specimens - a small sample of galena has a different weight that a large specimen of galena, but their densities are identical). Geologists actually don't normally use mineral density (I think it is because we don't like dealing with units). Instead, geologists use specific gravity. This property is officially defined as: the ratio of a mineral's density to the density of water (since the density of water is 1.0 g/cm3, all this really does is to dump the units). So where were we? Minerals containing heavy elements/ions will usually be more dense and will therefore have higher specific gravities than minerals composed of less dense elements/atoms. Here are some examples: Quartz: 2.65 Galena (PbS): 7.5 Silver (Ag) 10.0 Gold (Au) 19.3 (see image to right from www.csm.jmu.edu) You are not really expected to remember the specific gravities of the minerals, but you should recognize which minerals are "heavy" and which minerals are "light". Property 4: Hardness (talc, gypsum, graphite, quartz, corundum, diamond) Some minerals are hard, some minerals are soft and this is useful as a means of identification. Mineral hardness was first qualitatively employed by an Austrian-German mineralogist named Frederick Mohs. He grabbed a handful of relatively common minerals and arranged them in order of increasing hardness: Hardness Mineral Economic uses 1 (softest) Talc Baby powder 2 Gypsum drywall, Plaster of Paris 3 Calcite cement 4 Fluorite Toothpaste, steel manufacturing 5 Apatite Fluoroapatite = teeth 6 Orthoclase Not much; it looks pretty though 7 Quartz Glass, computer chips 8 Topaz gemstone 9 Corundum gemstone, abrasives 10 (hardest) Diamond gemstone, abrasives GY 111 Lab Notes D. Haywick (2006-07) 4 Note: the hardness scale is not absolute (refer to your lab manual for a good diagram illustrating this). Be careful where you scratch! Contamination may mislead you. Also note that streak plates have a hardness of 5.5 to 6 (may cause problems with streaking). You are not expected to carry a bunch of minerals around in your pocket to test mineral hardness. Instead, you can substitute other more common items: fingernail: H = 2.5 penny: H = 3.5 Glass: H = 5.5 to 6 Swiss Army knife: H = 6 Property 5: Luster (pyrite, muscovite, hematite, quartz, satin spar, bauxite, sphalerite) The way a mineral reflects light is called its luster. It is a very subjective property because it is described according to intensity and quality and nobody agrees about these. In general, there are 2 major types of luster: 1) metallic and 2) non-metallic. Some people also recognize a intermediary luster (sub-metallic) for those minerals that display lusters midway between metallic and non-metallic. Minerals that display metallic lusters reflect light like metals. They include galena, specular hematite, chalcopyrite, pyrite and real metallic minerals like copper, gold and silver) Submetallic minerals are more rare and the only ones that you have to worry about are muscovite and chlorite. The non-metallic minerals are the most abundant minerals. There are so many that it is necessary to use several luster subdivisions: Luster Description Mineral examples Metallic reflects light in the same way as a metal pyrite, galena Sub-metallic reflects light somewhat like a metal muscovite, chlorite Vitreous reflects light similar to a glass window pane (most quartz, fluorite, beryl common type of luster) (image to right from www.icminerals.com) Adamantine reflects light similar to a diamond (very intense) diamond, sulfur (rare) Resinous a luster having the appearance of resin or crystallized sphalerite tree sap Waxy a surface which look like it has been coated in wax or chert, milky calcite sugar glazing Non-metallic Pearly exhibits mother-of-pearl quality in how it reflects light gypsum Greasy has an oily quality in the way it reflects light talc Silky reflects light in much the same way as silk fiber gypsum (v.